Metal oxides are attractively applied in catalysis, energy storage, energy conversion, electronics, sensing, optics and functional devices. Nanostructured particles have engrossed both scientific and industrial practitioners due to their outstanding physical and chemical properties as well as their fascinating behavior. Excellent properties of nanoparticles are largely due to their large surface area to volume ratio, unusual adsorptive properties, surface defects and great diffusion coefficients.
Nanoscience research has so far been mostly related to development and synthesis of nanopowders and nanostructures. Powder synthesis with size, shape and size-distribution control has been a major part of colloid chemistry for decades. Compared to other methods of synthesizing metal oxides, liquid-phase routes (like sol-gel) have shown advantages like low-temperature requirement, metastable phase formation, morphology-control option and superior homogeneous composition attainment. Nickel oxide (NiO) usually considered as a model of p-type material—is a very promising semiconductor with wide band gap of 3.6-4.0 eV. It has attracted vast number of researchers because of its chemical stability, excellent and unique electrical, optical and magnetic properties, outstanding catalysis effect, gas sensing, chemical sensing (particularly as a negative electrode in Li-ion batteries and fuel cells), electrochromic behavior and magnetic and electrochemical super-capacitance.
Nickel oxide (NiO) has, in particular, behaved excellent when used as a catalyzer. Nanocrystalline NiO is expected to possess improved properties when compared to micrometer-sized NiO particles due to the volume effect, the quantum size effect, the surface effect and the macroscopic quantum tunneling effect.
There are several precipitation methods which exhibit simplicity, high yield and easy particle-size control. Research activities for synthesis of NiO nanoparticles via sol-gel, microemulsion precipitation, chemical vapor deposition and sputtering have progressed during past decade Specific surface area of a nano-NiO catalyst is much bigger than that of the conventional nickel-based (micro-NiO) catalysts. Nano-NiO particles, in particular, can be loaded on the surface of lower-cost carriers for saving purposes. It is expected, thus, that the nano-NiO catalyst can enhance the performance of biomass gasification/pyrolysis due to its higher activity.
Organic components and organic reaction pathways play a fundamental role in the nonaqueous synthesis of inorganic nanomaterials. A simple way to circumvent many problems of aqueous chemistry is to perform the synthesis procedure in organic solvents under exclusion of water. To enhance the performance of NiO for more important applications, the conditions of synthesis must be well-controlled to obtain ultra fine powders with a narrow particle-size distribution.
Solvent-directed processes involve the reaction of metal oxide precursor(s) with a common organic solvent and usually take place at lower temperatures (50-250° C.). Small number of reactants (precursor and solvent) makes it possible to study the chemical mechanisms involved in metal oxide formation through the characterization of the organic by-products. Homogeneous precipitation method is one of the economically feasible processes to prepare monodispersed metal oxide particles of various shapes and sizes.